A usual electric generator system includes a wound stator associated with a field structure that is energized with a D.C. current and rotated relative to the stator. One way of providing the direct current for energization of the field winding is to obtain alternating current energy from the generator stator windings by means of a stationary transformer, rectify the output current of the transformer and apply the resulting D.C. voltage to the field winding through a slip-ring and brush connections.
Another system comprises a dynamo electric machine involving a synchronous generator used with a brushless transformer exciter, an example of such a system being disclosed in U.S. Pat. No. 3,210,644 granted to Sparrow. That patent discloses a transformer exciter having a voltage generating rotor driven with the rotating field winding of the generator and operatively coupled with a stator winding or windings energized with alternating current in series with the load end or ends of a phase or phase windings of the generator stator. The current generated in the exciter rotor is rectified and the output of the rectifier applied to the generator field winding. Also, a voltage tap may be made between a point or points near the load end or ends of the generator stator winding or windings to additionally apply a voltage-responsive alternating current voltage to a part or parts of the stator winding or windings. Voltage regulation may be obtained by including a variable impedance winding in series or parallel with the exciter stator winding or windings.
Another brushless type of alternating current generator in combination with an exciter is disclosed in U.S. Pat. No. 3,344,338, also to Sparrow. In that patent, the exciter is energized with direct current from the AC generator stator. The exciter includes a D.C. main field winding energized through a full-wave rectifier connected across the load side of two phase windings of a three phase generator stator and also across a second full-wave rectifier connected to the secondary of a current transformer having its primary connected in series in the load end of one of the generator stator phase windings. The field core of the exciter is constructed to receive another winding energized from a voltage regulator at the output of the generator stator and the core is subject to saturation when this winding is energized to provide a suppressor type voltage control.
The first mentioned prior art construction requires a special potential current transformer and is subject to the disadvantages of slip ring constructions. The Sparrow U.S. Pat. No. 3,344,338 construction also has the disadvantage of requiring transformers. Also the rectifiers therein are subjected to high output voltages subjecting them to breakdown stresses. The device of Sparrow U.S. Pat. No. 3,210,644 requires a complicated exciter structure providing rotating transformer action.